The present invention, in general, relates to the application of various coatings to the surfaces of fine tungsten carbide particles so as to limit grain growth during the fabrication of useful articles from such particles, and otherwise improve the properties of such articles. Such composite particles are useful in fabricating better, more controllable, lower cost metal and/or ceramic parts using, for example, powder metallurgy techniques. The invention further relates to individual particles or agglomerates which have at least a second, discreet phase material, of different composition from the core tungsten carbide particles, applied to their surfaces, and to compacts made therefrom.
Tungsten carbide cutting tools are extensively used for many metal shaping and metal removal operations. Such cutting tools often take the form of tungsten carbide-cobalt (WCxe2x80x94Co) hardmetals. Those skilled in the art recognize that improvements in the properties of the WCxe2x80x94Co-based hardmetals immediately translate into improved efficiencies in the overall metal cutting and forming industries. Thus, the need for such improvements is well recognized.
A conventional method for the production of WCxe2x80x94Co-based hardmetals typically includes the bulk mechanical mixing of cobalt powder with tungsten carbide powder in a ball or attrition mill. The mechanical mixing is usually carried out in a liquid medium (usually volatile organic solvents) and includes the addition of lubricants such as waxes to aid in the subsequent pressing step. The mechanically mixed powders in the slurry form are usually spray dried to form agglomerated powder particles which can flow quite easily into the molds and dies used for pressing. The powders are typically pressed either in a uniaxial manner or in a cold isostatic press. Typically, the parts can be quite small (a few millimeters in thickness) to relatively large (around approximately an inch in thickness). The as-pressed green parts have lubricants in them. These lubricants are subsequently burned out using a long low temperature burnout cycle. The parts are subsequently heated to a temperature where liquid phase formation takes place and full density is achieved rapidly. The typical sintering temperature is around 1400xc2x0 C. Quite often, a low pressure is applied on the parts during the liquid phase sintering step. The WC grain size inevitably grows considerably during the sintering step, and the desired contiguity between the WC grains is compromised. As a result the WC grains are larger than desired, and some WC grains are directly touching one another.
Numerous grades of WCxe2x80x94Co-based hardmetals are used by the metal cutting industry. The grades have varying chemistries, WC grain sizes, and different mechanical properties. One of the proposals which has been made to improve the characteristics of WCxe2x80x94Co hardmetals is to decrease the WC grain size. Micrograin WCxe2x80x94Co hardmetals with a WC grain size around 1 Micron (xcexcm), as compared to some of the conventional carbide tools that have a WC grain size in the range of 25 to 50 microns have been proposed. It has been proposed that finer WC grain sizes might further improve the properties of the hardmetals. Submicron sized WC powders with WC particle sizes of from 0.8 to 0.2 xcexcm have been proposed. However, there are certain limitations to this approach.
Most mechanical properties such as hardness, transverse rupture strength, and fracture toughness will be dependent on the cemented carbide""s final microstructure, especially the tungsten carbide grain size, the matrix (cobalt) mean free path, and the contiguity of the tungsten carbide. The WC particles should not be touching one another. Material hardness generally increases with either finer WC grain size, or shorter cobalt mean free path. However, material strength reaches a maximum with decreasing grain size, below a certain average particle size, further grain refinement with bulk mixed materials results in a drop in material strength. This drop in the strength is generally due to increased tungsten carbide contiguity, that is, more grains are touching one another.
Other limitations of the conventional bulk processing approach, where bulk materials are milled together to achieve mixing of the WC grains with other ingredients, include non-uniform matrix distribution, segregation of light element impurities at the interfaces, and some significant grain growth that is associated with liquid phase sintering.
One of the first technical challenges that is encountered in processing submicron sized WC powders is the fact that grain growth invariably occurs during the liquid phase sintering process. Thus, the gains that are anticipated when starting with a fine WC powder are often offset by the grain growth associated with liquid phase sintering. It has been proposed to overcome this technical challenge by adding certain materials such as chromium carbide, tantalum carbide or vanadium carbide to the mixture. These additives act as grain growth inhibitors. Though the exact mechanism by which the grain growth inhibitors retard the WC grain growth during liquid phase sintering is not clear, it is postulated that the grain growth inhibitors quickly go into solution and interfere with the interfacial dissolution and reprecipitation of the WC, thus retarding the grain growth process. This expedient, by itself, limits, but does not entirely eliminate, WC grain growth. Thus, the final structure has a fine dispersion of tungsten carbide grains in which the grains are significantly larger than the starting WC grains.
Another technical hurdle is that the contiguity of the WC grains increases as the tungsten carbide grains become finer and finer. As contiguity increases, the desired mechanical properties degrade. This is a problem with bulk mixed materials that cannot be solved by conventional means, because the mean free path of the binder phase decreases with decreasing WC particle size, particularly when the materials are mechanically mixed in bulk.
Some of the other expedients that have been proposed to improve the performance of the carbide tools include the use of chemical vapor deposition or physical vapor deposition operations to deposit coatings of TiC, TiN, alumina, Ti(O,C), or the like, on conventional WCxe2x80x94Co-based bulk tool surfaces. For example, completed tungsten carbide tools are often coated with a hardphase such as alumina to lengthen the useful life of the tool. Also diamond like coatings have been applied to bulk tool surfaces. There is a large cost associated with the production of these surface coated cutting tools. Though these coated tools provide a tremendous benefit in the performance, the performance benefit lasts only as long as the coating is there. Once the coating has worn off, the tool cannot be easily re-sharpened and put to use. The current innovation does not deal with coating the bulk tool materials but rather with the coating of individual WC grains (several agglomerated grains may also be coated together) to attain a combination of several desired attributes.
Some of the other problems that impact on the properties of the hardmetal material are the formation of eta phase or free carbon within the structure, and the incorporation of light metal impurities at the interfaces between the particles. The performance of the carbide tool is primarily dependent on the hardness, transverse rupture strength (TRS), and fracture toughness of the material.
The present invention relates, among other things, to a process for forming coated tungsten carbide particles, the coated tungsten carbide particles so formed, and the formation of tungsten carbide (WC) multigrain compacts from such coated particles. The surfaces of the particles are modified with one or more coatings to improve the processability, handleability, physical, chemical and/or other properties of the multigrain compacts. The coatings are applied with the objective of limiting the grain growth of WC during the formation of a multigrain compact.
By ensuring, through the pre-coating of the WC grains, that each WC grain in a compact is completely surrounded by matrix material, significantly lower cobalt content can be used without a concomitant decrease in the toughness of the carbide compact. The benefit can be especially improved if cobalt coated sub-micron sized WC powders are used, and the compact is formed at low enough temperatures that the processing is all solid state. That is, the WC grains, according to one embodiment of the invention, are never subjected to a processing temperature at which they grow rapidly. Alternatively, the WC powders can be liquid phase sintered without significant grain growth through the incorporation at the particle level of diffusion resistant layers that are so highly resistant to diffusion that they prevent or at least significantly limit micro-structural rearrangement.
According to one embodiment of the present invention, better dispersion of grain growth inhibitors is provided at the particle level. The effectiveness of the grain growth inhibition is thus significantly improved. Other hardphase materials such as, for example, TiN, TiC, Ti(N,O), alumina, and the like, that are conventionally used for coating bulk tools, are applied to the individual particles at the particle level. As a result, such other hardphase materials are incorporated throughout the microstructure of the compact itself at the grain level. The performance of the hardmetal material is thus significantly improved. The current invention provides unique coated tungsten carbide particles that afford a number of improvements in the processing of micro-grain carbides and in the resultant compacts.
Coated microgram carbides for use according to the present invention can be produced, for example, by the use of recirculating, fast fluidized bed chemical vapor deposition techniques and equipment, for example, as described in Sherman et al. U.S. Pat. No. 5,876,793. Such recirculating, fast fluidized bed chemical vapor deposition techniques and equipment are used to deposit various different materials on WC micrograms to produce a unique composite powder. When consolidated, this powder results in a unique WCxe2x80x94Co-based hardmetal with an exceptional combination of properties. The fluidized bed is a highly efficient solid-fluid contacting device that is well suited to coating particles, enabling numerous benefits to be achieved through more precise microstructural and compositional control of materials.
In the case of fine powders that fall in the category of Geldhart Class C type particles, surface phenomena tend to dominate their behavior. This is because of their high surface-to-volume ratio. In such cases, van der Waals, electrostatic, and surface tension forces often have a dominating effect on the properties. Handling of these particles is extremely difficult. High aspect ratios, such as found in case of fine whiskers or submicron powders, further aggravate the handling problem. Modifying such fine submicron WC particulates by micro-encapsulating them with different materials is generally very difficult. One way to overcome these problems is to fluidize these fine particles in the turbulent and fast-transport regimes, where the high gas shear forces and massive turbulence tend to nullify to a great extent the cohesive effects of the fine particulates. Another advantage of operating in the turbulent and fast-transport regime is that it allows high gas shear forces and particle collision forces to continually break up agglomerates as they form. While so fluidized the particles are microencapsulated with one or more materials using chemical vapor deposition techniques. It has been determined that operating a recirculating fluidized bed in the fast-transport operating regime (which is above the transport velocity) or in the turbulent fluidization regime enables fine particles including fine powders, whiskers, chopped fibers, and the like, to be fluidized with high product yields. WC powders in the submicron range can thus be microencapsulated.